Rail Engineer • June 2016
Applications for modular multilevel converters
43
JAMES GOULDING
ELECTRIFICATION/POWER
N
ew and more efficient power transmission systems are being developed to support the UK’s rail infrastructure that will bring a range of installation, operational and environmental benefits. Sustainability, security and continuity of the power supply remain crucial requirements for any rail solution, and the Siemens development team has produced a number of reactive power compensation systems to address these issues. The most recent is an innovative static variable compensation system (called SVC Plus®), which is a universal solution for grid enhancement. Using modular multilevel converter (MMC) technology, the SVC Plus system provides a number of benefits compared to conventional reactive power compensation solutions. Importantly, the system significantly reduces the risk of voltage collapse or blackout, improved dynamic stability of the transmission system and increased power quality - all of which are critical requirements for the rail sector. It also provides environmental benefits, with low harmonic generation and low noise emissions. And, by incorporating voltage-sourced converter technology, the SVC system delivers a nearlyperfect sinusoidal-shaped AC waveform, meaning that there is only little (if any) need for high frequency filtering, and no need at all for low-order harmonic filtering.
Flexible and relocatable Because it uses a relatively low number of proven and robust standard components, MMC technology provides a high degree of flexibility in converter design and station layout, simplifying the design, planning, installation and commissioning processes, as well as any subsequent engineering tasks. For rail infrastructure owners and operators, this means it is fast and efficient to install, with simpler and more cost-effective maintenance and reduced service requirements. The system’s flexibility extends to its housing, with both conventional building and containerised solutions available. This smaller physical footprint reduces the turnkey costs and makes this an ideal option where trackside space is limited and/or costly. Modularity also means that the system is both expandable and relocatable as requirements change over time. Its open-rack configuration
enables grid connection up to 36 kilovolts (kV) and +/-100 megavolt-ampere reactive (MVAr) units to be made without transformers. This modular approach makes SVC Plus uniquely adaptive without compromising on performance, construction time or cost-effectiveness. Hybrid solutions with mechanically switched capacitors (MSCs) or reactors (MSRs) are also available, and for higher system voltages, standard AC transformers are used. MMC technology also means that harmonic generation emission levels are quite low, with high frequency noise being absorbed by small high frequency blocking filters. This results in an SVC design which is practically independent from individual network impedances. Operationally, the MMC-based SVC system provides very high levels of system availability, thanks to the use of standard control and protection hardware and software. Providing a stable network, the system also delivers very fast response times and excellent under-voltage performance, with highly efficient voltage support. The system also provides rail infrastructure owners with significant ongoing economic benefits, with low switching frequencies (and therefore fewer losses), fewer components (resulting in less time and reduced cost demands for planning, engineering, construction and commissioning) and lower space requirements (reducing land and property costs).
Three phases into one In addition to the SVC solutions, MMC technology is also at the heart of Siemens’ static frequency convertor (SFC) systems, which supply single-phase traction power networks from threephase networks. These solutions are supplied under the proven Sitras® name. In essence, SFC systems consist of only one converter that directly couples two networks - the three-phase AC voltage is directly converted into a single-phase AC voltage with different frequency.
Due to the multilevel technology, no traction transformer is needed to feed the overhead contact line. These innovative multilevel traction converters are quiet, space-efficient and require minimal maintenance. Covering both central and decentralised traction power supply systems, SFC systems provide a high degree of efficiency over the entire operating range, optimising the use of the primary energy and delivering high availability. Again, a modular converter provides the flexibility for each system to be adapted to meet the rail operator’s specific requirements, with block capacities of 12 to 120 megawatts (MW) and the ability to connect multiple blocks in parallel. As a result, a total power rating of up to 600MW is attainable. With overload capability, the short-circuit current is significantly higher than nominal current and the system delivers positive supply system reactions. This is due to a number of factors: the generation of an output voltage with small voltage steps (comparable to that of a generator); no extra filters being needed on the three-phase side and railway side and the power recovery function being provided without additional equipment. And with no complementary energy storage units, the stored energy is distributed to a large number of power modules, so any fault remains limited to the easy-to-replace power module. Already used in rail applications across Europe and Australia, as well as on the Channel Tunnel Rail Link, SVC and SFC systems are increasingly being considered for both mainline and metro applications - in the UK and elsewhere. Given their flexibility, cost effectiveness and the range of performance benefits they deliver, these systems look set to become a more common solution for infrastructure operators. James Goulding is business development manager, Siemens Rail Electrification